Recent improvements in combine harvesters have included microprocessor controls for automatically adjusting various combine operating parameters such as rotor speed, fan speed, concave clearance, chaffer sieve opening and cleaning sieve opening. It has been found that when certain parameters such as the cleaning sieve opening and chaffer sieve opening are placed under microprocessor control, damage to the sieve or chaffer slats may result if the microprocessor attempts to close the cleaning sieve or chaffer sieve opening while there is crop material or some foreign object such as a stone therein.
Another problem encountered with the use of a microprocessor to adjust the cleaning sieve (hereinafter referred to as the sieve) or chaffer sieve (hereinafter referred to as the chaffer) occurs because of play or backlash in the mechanical linkages or between the actuators and the sieve or chaffer. This problem destroys the accuracy of a control system where it is desired to make the adjustment of the chaffer and sieve openings to an accuracy of less than 0.030 inch.